Case Studies - PTTC Rockies

Geochemical wireline logging provides a real-time, quantitative characterization of coal bed lithology and mineralogy that can estimate total gas volume and degree of cleating in coal beds. Unlike conventional logging methods, a geochemical log directly measures coal and ash mineralogy based on their chemical makeup and these measurements are relatively unaffected by borehole conditions, obtainable in fluid- or air-filled, open or cased boreholes-providing a more accurate and reliable estimate of coal gas volume and cleating. Thus, important information about CBM resources and producibility are derived in-situ-at discrete depths or averaged over one or many coal beds.

The ECS* Elemental Capture Spectrometer sonde and RST* Reservoir Saturation Tool are routinely employed for quantitative geochemical and lithologic characterization in conventional oil and gas reservoirs. These tools use prompt gamma ray neutron activation spectroscopy to analyze for Ca, S, Si, Fe, Ti, and Gd. The processing has been modified to include coal, based on a spectral H measurement that is highly sensitive to coal content. The result is a continuous log of coal plus carbonate, pyrite, clay and sand weight% that characterize the depth, thickness, net footage, and mineral content of coal seams, as well as the lithology throughout the rest of the logged borehole.

No logging tool can directly measure the gas adsorbed to coal. Instead, gas content is derived by correlating coal properties measured with logs to gas content, based on representative core analyses. In-situ gas content in coals is estimated from log measurements by the following empirical steps:

Proximate analysis components of coal (fixed carbon, volatiles, and moisture) are determined from the geochemical log of total mineral content weight%, using a relationship derived from proximate analyses performed on core. (This step can be eliminated if density and neutron porosity measurements are available, enabling direct delineation of fixed carbon and volatiles from logs.) While ash, fixed carbon, and volatile volume% can be derived from density and Pe (or neutron porosity) logs alone, this requires assumptions on the bulk density and Pe (or neutron porosity) of each component-particularly difficult for ash. With the addition of geochemical logs, these assumptions are significantly reduced, because the total coal and each mineral constituent are accurately measured.
The total gas content and gas adsorption isotherm are determined at discrete depths from the coal rank (fixed carbon versus volatiles) and ash weight%, using an empirical relationship derived from proximate analysis and gas desorption/adsorption tests performed on core. The relationship is derived based on a physical gas adsorption model, such as the Langmuir equation. Also required for the gas adsorption model are estimates of in-situ pore pressure and, secondarily, temperature. Gas content is directly proportional to coal weight%, because gas does not adsorb to the ash minerals. Because geochemical logs provide a more accurate and robust measurement of coal weight% than density logs alone, the resulting estimate of gas content is more reliable.

While general models for the above empirical relationships have been developed using large core data sets from various coal basins, the relationships produce the most accurate results where they are calibrated to local coal, using core analysis results from wells in the area. This calibration is typically only required once for a specific area, after which gas content can be estimated solely from the logs.

An indication of the degree of cleating at discrete depths in coal beds is provided by the geochemical log mineral ash measurements (carbonate, pyrite, clay and sand). Core and log data observations and well performance data have empirically shown that a higher volume of clastic minerals (e.g. quartz and clay) is detrimental to cleat development in coal, by holding the coal together. Conversely, small volumes of secondary minerals (e.g. calcite and pyrite) are good indicators of well-developed cleating, resulting from mineral deposition from water flowing through the coal. Based on these observations, relationships using cutoffs on the mineral ash volume measurements have been developed to determine whether coal beds are poorly, partly, or well cleated. These relationships can be refined for local basins, if similar core and well performance data are available. In a recent Indian study, quantitative estimates of cleat porosity were also made from geochemical logs by deriving an empirical relationship between mineral concentrations and an independent measurement of cleat porosity. In turn, the inferred degree of cleating or cleat porosity provides an indication of gas producibility of the coal at that depth.

Compared to the conventional approach for coal bed methane evaluation, where density is typically the primary logging measurement, geochemical logging provides:

a more accurate, reliable measurement of coal content and, thus, total gas content;
a precise measurement of coal bed mineralogy, enabling an estimate of cleating-an important indicator of gas producibility; and
the option for cased hole evaluation.

In turn, this evaluation enables the optimized well completion, stimulation, and production of coal bed methane. The measurements can be enhanced by running the geochemical probe in combination with a density-Pe probe (or high resolution neutron porosity probe in cased holes), improving the vertical resolution of the geochemical measurements in coal beds from 20 to 8 inches or less and providing a direct measurement of coal rank.